2003 Chevy Impala Engines hesitates above 3500 RPM and loos

2003 Chevy Impala, 3800 engine, hesitates at 3500 rpm looses power and will not accelerate?
3.8 L engine approx 100,000 miles. Changed fuel filter and fuel pressure is about 48 PSI. If you accelerate the engine while not in gear it will rev to approx 3500-3800 RPM then start to hesitate and shutter and will decelerate to below 3000RPM and as you continue push on gas pedal it continues to loose power. If driving the vehicle it runs till about 30 MPH then starts to loose power and will not accelerate. We did get 2 codes P0301 and P0502. These were not consistently showing. The engine idles fine with no missing and runs fine till about 3500 RPM and then it does not miss just hesitates and looses power.
Any ideas would be greatly appreciated.
Thank you for any help!